There is a high incidence of infectious disease in heroin-dependent individuals. In spite of the major health issues surrounding heroin use, few studies have examined the impact of heroin on immune status. A wealth of recent data has revealed that the induction of nitric oxide plays a critical role in the immune response to infectious diseases. New data from our laboratory has provided the first evidence that heroin administration produces alterations of inducible nitric oxide synthase (iNOS), the enzyme responsible for the production of nitric oxide. Specific Aim I extensively tests the hypothesis that heroin induces widespread alterations in the in vivo expression of NOS. This specific aim fully evaluates the effect of heroin on lipopolysaccaride (LPS)-induced iNOS expression. The experiments establish dose-effect relationships for heroin, as well as the antagonism of those effects with naltrexone. To measure NOS production in vivo, we employ both RT-PCR and western blotting to measure alterations of iNOS mRNA expression and protein in spleen, lung, and liver tissue. We also measure nitrite/nitrate in plasma. Specific Aim II tests the hypothesis that heroin-induced alteration of NOS production is conditioned to environmental stimuli paired with drug administration. The demonstration that immune alterations can be induced by the stimuli associated with heroin administration is important for it indicates that the detrimental health consequences of heroin use are conditioned to environmental stimuli and not solely the pharmacological property of the drug. The proposed studies determine whether heroin's effect on nitric oxide production can be elicited by environmental stimuli paired with heroin administration. Control procedures are used to assure that these conditioned alterations are due to conditioning processes per se, and not the result of ancillary factors involved in the conditioning procedure. Tests of extinction and latent inhibition will establish that the effects follow the principles of Pavlovian conditioning. Specific AimIII tests the hypothesis that drug self-administration is an important behavioral variable controlling heroin's effect on the production of nitric oxide. The self-administration paradigm provides a clinically relevant methodology for the assessment of heroin's effects on nitric oxide production. The study compares the effects of heroin self-administration to those of passive drug administration. Investigation of the effects of self-administered heroin on nitric oxide production is driven by evidence showing that control over drug administration greatly influence it's physiological effects. The demonstration that heroin is differentially efficacious in altering nitric oxide production depending upon whether the drug is self-administered or passively infused has important implications for the health consequences of heroin use. Collectively, the characterization of heroin-induced modulation of inducible nitric oxide synthase production provides a greater understanding of how opioids impact the immune system and health.